Switched mode power supplies (SMPS) are widely used for supplying electronic loads to devices such as computers, television sets or any other electronic appliances with a suitable voltage level out of a mains voltage in the range of typically e.g. 90V to 240V rms. Transformers in many cases galvanically decouple the load from the mains and provide the appropriate voltage level at the secondary side consume less space and are more economic if designed for higher frequencies, above the frequency range from 50 Hz to 60 Hz of most AC mains. Electronic switches, e.g. high-voltage MOSFETs or IGBTs are commonly used for providing a primary side of the transformer with appropriate high frequency voltage and current waveforms out of a direct current (DC) link which can be generated by rectifying the mains voltage. It is noted here that the use of such electronic switches is not restricted to the applications mentioned above.
In an electronic switch connected to the primary side of the transformer, when driving the primary side of the transformer with high voltage, high frequency and high current signals, both ohmic losses and switching losses occur. These losses are present when turning-on and especially when turning-off the electronic switch. To reduce the switching losses and at the same time increase an overall efficiency, a number of configurations and methods for driving the transformer have been established. These methods include an operation of a resonant tank wherein the electronic switches mainly are turned-on and/or turned-off close to or at zero crossing of the voltage. Thereby, switching losses can be reduced. Such methods are often referred to as soft switching of the electronic switches. In normal operation, the maximum voltage that is applied to the load terminals during soft switching is the voltage of the DC link. Situations may occur, however, where soft switching conditions are not achieved, e.g. during power-up of the SMPS, load jumps, etc. In these cases, the electronic switch cannot necessarily turn-off close to zero voltage or zero current but at a significant current and/or voltage at the terminals of the electronic switch. In these cases a significant voltage overshoot can occur which exceeds the DC link voltage.
Therefore, conventionally used electronic switches provide a blocking capability which is exceeding the voltage of the DC link. A charge stored between the load terminals of the high-voltage electronic switch, however, increases with increasing blocking requirements of the device. This can adversely affect both the switching losses of the high-voltage electronic switch and the control stability of the SMPS.
The on-state losses of a high-voltage electronic switch having a given chip area can significantly increase with increasing blocking requirements. On the one hand, a width of a drift zone sustaining the voltage across the load terminals will linearly increase with the blocking capability, according to a first order approximation. Furthermore, a reduction of a net doping of the drift zone can be provided in order to improve the blocking capability. As a result, the on-state losses of a high-voltage electronic switch can increase significantly with increasing blocking capability, e.g. the on-state losses of the high-voltage electronic switch can increase disproportionally high with increasing blocking capability.
In view of the above, there is a need for improvement.